A. Field of Invention
The present invention is related to wireless communication systems, and more particularly, to a method and system of utilizing Walsh codes in a sectorized CDMA cellular communication system having multiple beam zones.
B. Description of Related Art
In a typical wireless communication system, an area is divided geographically into a number of cell sites, each defined by one or more radiation patterns created by an emission of radio frequency (RF) electromagnetic (EM) waves from a respective base transceiver station (BTS) antenna. Each cell site is typically further divided into two, three, or more sectors, where the sectors provide radio coverage for a selected area within the cell site. Each sector of the cell typically uses dedicated antennas to provide the required coverage.
In CDMA communication systems, each sector uses a unique PN code (commonly referred to as a short PN code offset) to distinguish itself from surrounding sectors and cells. Within each sector, channels are distinguished by yet another code, termed a Walsh code. In an adjacent sector, the Walsh codes may be reused because channel separation is provided by a different offset of the short PN code for that sector. Thus, the number of available forward channels (BTS to MS) on a given carrier frequency in a sector is limited by the number of available Walsh codes. In the ANSI/TIA/EIA-95-B-99 standard entitled “Mobile Station-Base Station Compatibility Standard for Wideband Spread Spectrum Cellular Systems” (published Feb. 1, 1999), the contents of which are incorporated by reference herein, there are sixty-four available Walsh codes, while in CDMA 2000 series (TIA/EIA IS-2000 Series, Rev. A, published Mar. 1, 2000), one hundred twenty-eight Walsh codes are available. Both of the ANSI/TIA/EIA-95-B-99 and the TIA/EIA IS-2000 Series, Rev. A, standards are incorporated herein by reference, and are available from the Telecommunication Industry Association, 2500 Wilson Boulevard, Suite 300, Arlington, Va. 22201.
The maximum number of forward channels for a single RF carrier is therefore fixed in a given CDMA cell site having a particular sector topology. To increase system capacity, quasi-orthogonal functions (QOF) may be used to supplement the available Walsh codes. QOFs are derived from Walsh codes, and are not perfectly orthogonal to the set of Walsh codes. Nevertheless, they provide a relatively low level of inter-user cross-correlation interference that makes them suitable for use when extra capacity is needed. Techniques of generating QOFs are well known, and are based on a mask value and masking function, from which an entire set of QOFs may be generated by masking the original set of Walsh codes. Accordingly, each QOF mask may be used to generate a QOF set.
Oftentimes, however, the capacity of the system is reached before all of the Walsh codes have been used due to interference between the CDMA users in a sector or nearby sectors. To reduce interference, smart antennas may be used to limit the amount of signal power received from interfering users.
A smart antenna may actually be an array of antenna elements working together to produce a particular radiation pattern. Each antenna in the array is referred to as an antenna element (or simply an element). An antenna radiation pattern is also referred to as an antenna-beam or beam zone. A beam width of an antenna is a measure of directivity of an antenna and is usually defined by angles where the radiation pattern reduces to one half of its peak value or more commonly referred to as 3 db points (i.e., 3 decibel power level). Using sophisticated antenna arrays, a given sector may be divided into directional sub-sectors covered by one or more beam zones.
The use of smart antennas to reduce interference may allow an increase in system capacity. However, the number of available Walsh codes in conjunction with the QOFs, may still limit a sector's capacity. Consequently, a system that overcomes these limitations is desirable.